Subsea pipeline system

ABSTRACT

The invention relates to a subsea pipeline system for connecting two hydrocarbon production, storage and/or processing structures. The subsea pipeline has a first duct section supported on the seabed, a bridging duct section extending across a lower seabed part, at a distance from the sea bed, and a second duct section supported on the sea bed. The first and second duct sections are connected to the bridging duct section via an articulation joint allowing movement of the bridging duct section in a length direction of the first and second duct sections. A first tensioning element exerts an upward force on the bridge duct sections and a second tensioning element exerts a downward force on the bridge duct section. The first and second tensioning elements include a mutual angle to exert a horizontal pulling force on the bridging duct section.

The invention relates to a subsea pipeline system for connecting twohydrocarbon production, storage and/or processing structures.

From WO 99/62762, in the name of the applicant, a submerged transferpipe system is known for transfer of hydrocarbons between two floatingoffshore structures at sea level. The known transfer pipe systemcomprises a horizontal duct section, which may have a length of 1000 mor more, suspended from inclined vertical duct sections on each of therespective offshore structures (SPAR buoy, FPSO, export buoy, etc.). Atensioning weight, or tensioning cable connected to the sea bed, isconnected near the flexible joints connecting the vertical an horizontalduct sections, such that a tensioning force is exerted on the horizontalduct which prevents buckling or bending and which reduces horizontaldisplacements due to currents acting on the horizontal duct section.

The known pipeline structure is particularly suitable for use inmid-depth waters such as depths of 50-500 m.

It is an object of the present invention to provide a transfer pipesystem which is suitable for use on an irregular seabed.

Hereto the transfer pipe system according to the present inventioncomprises a first duct section supported on the sea bed, a bridging ductsection extending across a lower seabed part, at a distance from the seabed, and a second duct section supported on the sea bed, the first andsecond duct sections being connected to the bridging duct section via anarticulation joint allowing movement of the bridging duct section in alength direction of the first and second duct sections, wherein a firsttensioning element is with a first end connected to the bridging ductsection at or near the articulation joint and with a second end to alifting structure which is located at a relatively short distance abovethe bridging duct section, for exerting an upward force thereon, and asecond tensioning element connected with one end to the bridging ductsection and with the other end to a downward pulling structure, which islocated below the bridging duct section, the first and second tensioningelements including a mutual angle such as to exert a horizontal pullingforce on the bridging duct section.

By placing the duct sections on the seabed, they are maintained in astable position without being subject to displacements by currents. Atthe location of lower regions of the seabed, such as subsea canyons, thebridging duct section is subject to horizontal drift displacement and tovertical displacements. By providing the lifting structure near the endparts of the first and second duct sections, and the downward pullingstructure, the ends of the bridging duct section can be firmly kept inposition near the higher sea bed part. The lifting structure does notextend more than half the water depth above the seabed. As thearticulation joint allows movement of the bridging duct section in itslength direction, sufficient possibilities for movement of the bridgingduct section are provided with respect to the fixed end parts of thefirst and second duct sections, such that buckling an bending of thebridging duct section is prevented.

The articulated joints furthermore allows easy positioning andinstallation of the bridging duct section as well as removal ortemporary disconnection of the bridging duct section for replacement andfor maintenance.

The first and second production, storage and/or processing structuresmay be on shore or offshore structures, located on or below the seasurface, such as SPAR buoys, FPSO's onshore power plants, LNG tankers,regasification plants, processing equipment, etc.

An embodiment of a subsea pipeline structure according to the presentinvention comprises a lifting structure that is supported on the seabedand which extends above the bridging duct section, the first tensioningelement comprising a line connected to the lifting structure and to thebridging duct section, the line making an angle with the verticaldirection. The lifting structure may be a frame-construction or tower,preferably with a height between 5 and 50 m. The tower provides a securefixing of the end parts of the first and second duct sections on the seabed without being affected by subsea currents.

In another embodiment, the lifting structure comprises a buoyancy deviceexerting an upward force on the ends of the bridging duct section.

The articulation joints allowing movement of the bridging duct sectionin the length direction of the first and second duct sections, as wellas transversely to the duct sections in a horizontal and verticaldirection, may comprise a flexible jumper hose or a hard pipe sectionwith a pipe swivel, a flexible joint, a bellow or a ball joint on eachend of the pipe section. dr

Some embodiments of a subsea pipeline system according to the presentinvention will be described in detail with reference to the accompanyingdrawings, by way of non-limiting example. In the drawings:

FIGS. 1 and 2 show a schematic view of an embodiment of aseabed-supported pipeline system according to the present invention, thebridging duct section being supported by a tower;

FIGS. 3-5 show a schematic view of an embodiment of a seabed-supportedpipeline system, the bridging duct section being supported by a buoyancyelement; and

FIGS. 6 and 7 show embodiments of an articulation joint comprising hardpiping and swivel elements.

FIG. 1 shows a submerged pipeline system 1 comprising two substantiallyhorizontal first and second duct sections 2, 3 supported on asubstantially horizontal part of the seabed 4. Near a lower seabed part5, which may for instance comprise a subsea canyon, the duct sections 2,3 are connected to a bridging duct section 6 via articulation joints 7,8. Near the edges of the lower seabed part 5, the bridging duct section6 is supported by a tower or frame construction 9, 10, which is anchoredin the seabed. A tensioning element 12, 13, such as a steel or polyestercable or chain, is with a first end 14 connected to the bridging ductsection 6 and with a second end 15 to an upper part of the tower. Thecable 12 extends at an angle α with respect to the vertical. A secondtensioning element, which is in this case formed by a weight 17, 18, issuspended from a cable 19, 20. The articulation joints 7, 8 are formedby flexible duct sections or flexible jumper hoses which can bend andflex to allow displacement of the bridging duct section 6 in the lengthdirection L as well as in a vertical direction in the plane of thedrawing or in a horizontal direction perpendicular to the plane of thedrawing due to drift caused by subsea currents. Movement of the bridgingduct section 6 is isolated from the stationary and rigidly supportedpipe sections 2, 3 and bending moments and stresses are not transferredto the horizontal duct sections 2, 3. The weights 17, 18 in combinationwith the inclined cables 12, 13 result in a horizontal tensioning forceon the bridging duct section 6, which is stabilised in this way and bywhich bending and buckling of the duct section 6 is prevented. Thehorizontal duct section 6 may have a length of between 50 and 1000 m.The length of flexible duct 7, 8 may be between 5 and 50 m. The heightof towers 9, 10 may be between 5 and 50 m. The bridging duct section maycomprise a supporting frame 25 along which a bridging duct 26 issupported. The frame 25 may comprise buoyancy members, such as toprovide neutral buoyancy to bridging duct section 6.

The horizontal duct sections 2, 3 may connect to subsea hydrocarbonstructures, such as wellheads, distribution manifolds, or may beconnected to offshore constructions at sea level, such as LNG-tankers,FPSO, SPAR-buoys, and the like. The ducts may also connect to onshorehydrocarbon processing and/or storage structures, such as gas fuelled oroil fuelled power plants, on shore gas or oil storage tanks, processingplants and the like. One of the duct sections 2, 3 may be connected toan onshore hydrocarbon storage and/or processing structure whereas theother one is connected to an offshore construction.

FIG. 2 shows a construction in which the lower tensioning element iscomprised of a cable 30 connected to a counter weight 31 via a sheeve32. In this way chances of interference of the weight 31 with pipeline2, 3 or with bridging duct section 6 are minimised.

In the embodiment of FIG. 3, a relatively short supporting frame 9 isused. The first tensioning element comprises a buoyancy device 36,attached to bridging duct section 6 via cable 37. The bridging ductsection 6 is with one end connected to a downward pulling cable 38,which is fixed in a connection point 39 to the seabed 4.

In the embodiment of FIG. 4, a buoyancy device 36 is connected via anarticulated joint to the end part of duct section 2 and is anchored witha cable 38 and anchoring element 39 to the seabed 4. Via a second cable41 extending at an angle to the vertical, buoyancy device 36 isconnected to the end part of bridging duct section 6, which carries atits end a weight 17. In this construction, a single buoyancy device 36supports both the end part of horizontal duct section 2 and the bridgingduct section 6.

In the embodiment as shown in FIG. 5, a relatively long downward pullingcable 38 is employed compared to the embodiment of FIG. 3, such that thedegree of movement of bridging duct section 6 is relatively large. Byuse of such a longer cable 38, the axial forces acting on the fluidtransfer duct 6 and can be adjusted to the environmental conditions bychange in length. Hereby, the stiffness of the system can be adjustedand optimised even after installation, or may be re-adjusted after acertain amount of time.

In the embodiment shown in FIG. 6, the articulation joint is comprisedof a rigid pipe section 40, comprising at its end part a pipe swivel orball joint 41, 42, which allows for movements perpendicular out of theplane of the drawing. In this construction, movement out of the plane ofthe drawing is restricted. By the use of rigid pipe sections, thepressures in relatively deep waters can be taken up.

In the embodiment shown in FIG. 7, the articulation joint comprises twojoint part 45, 46 and three pipe swivels 47, 48 and 49. In thisconstruction, horizontal movement of the bridging duct section 6 can bebetter accommodated compared to the construction shown in FIG. 6.

1-11. (canceled)
 12. A subsea pipeline system for connecting two structures comprising a first duct section supported on the sea bed, a bridging duct section extending across a lower seabed part, at a distance from the sea bed, and a second duct section supported on the sea bed, the first and second duct sections being connected to the bridging duct section via an articulation joint allowing movement of the bridging duct section in a length direction of the first and second duct sections, wherein a first tensioning element is with a first end connected to the bridging duct section at or near the articulation joint and with a second end to a lifting structure which is located at a relatively short distance above the bridging duct section, for exerting an upward force thereon, and a second tensioning element connected with one end to the bridging duct section and with the other end to a downward pulling structure, which is located below the bridging duct section, the first and second tensioning elements including a mutual angle such as to exert a horizontal pulling force on the bridging duct section.
 13. The subsea pipeline system according to claim 12, wherein the lifting structure is supported on the sea bed and extends above the bridging duct section, the first tensioning element comprising a line connected to the lifting structure and to the bridging duct section, the line making an angle with the vertical direction.
 14. The subsea pipeline system according to claim 12, wherein the lifting structure comprises a buoyancy device, connected with a line to the bridging duct section.
 15. The subsea pipeline system according to claim 12, wherein the second tensioning element comprises a line connected to the bridging duct section and to a connection point at or near the sea bed, the line making an angle with the vertical.
 16. The subsea pipeline system according to claim 12, wherein the downward pulling structure comprises a weight suspended from the bridging duct section.
 17. The subsea pipeline system according to claim 12, wherein the ends of the first and second duct sections are supported on the sea bed via a respective vertical support structure.
 18. The subsea pipeline system according to claim 12, wherein the articulation joint comprises a flexible duct section.
 19. The subsea pipeline system according to claim 12, wherein the articulation joint comprises a duct section extending at an angle with the first and second duct sections, and comprising near its respective ends a swivel.
 20. The subsea pipeline system according to claim 12, wherein the bridging duct section is provided with buoyancy such as to have a substantially neutral buoyancy.
 21. The subsea pipeline system according to claim 12, wherein the first and second duct sections and the bridging duct section are made of hard pipe.
 22. The subsea pipeline system according to claim 12, wherein the bridging duct section comprises at least two parallel duct sections.
 23. The subsea pipeline system according to claim 12, wherein the two structures are hydrocarbon production, storage and/or processing structures. 